In recent years, there has been a significant increase in the production of crystalline silicon solar cells that use plates of monocrystalline silicon or polycrystalline silicon as crystalline silicon substrates. Such solar cells have electrodes through which the generated electricity is extracted. In the conventional crystalline silicon solar cells, the electrodes are formed using conductive pastes containing conductive powders, glass frits, organic binders, solvents and other additives.
For example, Patent Document 1 describes a conductive paste used in the formation of electrodes on p-type silicon semiconductor substrates. This conductive paste includes 60 to 90 wt % of solids and 10 to 40 wt % of an organic vehicle based on the weight of the whole of the paste, the solids including 85 to 98.5 wt % of a silver powder, 0.5 to 10 wt % of an aluminum powder, and 1 to 10 wt % of a glass frit based on the weight of the whole of the solids.
Patent Document 2 describes a solar cell having a silicon semiconductor substrate (1), a surface-side diffusion layer (2) on the substrate, and an electrode (4) formed in contact with the diffusion layer by the application and heat treatment of a conductive paste (3) in which the main material includes at least one of silver and copper. This solar cell is characterized in that an aluminum-silicon alloy is added to the main material of the conductive paste (3).
Patent Document 3 describes a conductive paste for back-contact solar cells. Specifically, Patent Document 3 describes that the conductive paste includes (a) conductive particles including silver particles, and additive particles selected from (i) metal particles selected from the group consisting of Mo, Tc, Ru, Rh, Pd, W, Re, Os, Ir and Pt particles, (ii) alloys including the above metal particles and (iii) particles loaded with the above metal particles, (b) a glass frit, and (c) a resin binder. The additive particles additionally used in the conductive paste of Patent Document 3 are relatively expensive metals.
For example, the glass frit used in conductive pastes for the formation of electrodes in the conventional solar cells, in particular crystalline silicon solar cells, is lead borosilicate glass frit containing lead oxide.